The refrigeration apparatus to which this invention relates comprises a screw compressor that is both cooled and lubricated by the circulation of oil therethrough. The oil issues from the compressor in a mixture with compressed refrigerant, and that mixture is delivered to an oil separator, from which the separated oil is returned to the compressor through an oil pump. The compressed refrigerant passes from the oil separator through a condenser to a high pressure receiver in which it is held for circulation through the evaporator or cooling coils of the system.
The above mentioned U.S. Pat. No. 4,275,570, which has a common assignee with the present application, discloses improved means for cooling the compressor lubricating oil, whereby the need for a separate oil cooling heat exchanger is eliminated. According to that patent, a small pump for liquid refrigerant is provided that has its inlet connected with the receiver and has its outlet communicated with the discharge duct that carries mixed oil and refrigerant from the compressor to the oil separator. The mixture of oil and compressed refrigerant is cooled by the liquid refrigerant which this pump introduces into the discharge duct; hence the refrigerant pump and its associated connection not only effect the necessary cooling of the lubricating oil but also greatly improve the performance of the oil separator and desuperheat the compressed refrigerant.
An important feature of the apparatus of U.S. Pat. No. 4,275,570 is provision for controlling the rate of delivery of liquid refrigerant from the high pressure receiver to the discharge duct, so as to match that rate to the prevailing output of the screw compressor. Such control ensures delivery of enough liquid refrigerant to afford adequate oil cooling but not so much as to cool the refrigerant to its saturation temperature and thus cause formation of drops of liquid refrigerant that would be separated out in the oil separator and would subsequently cause cavitation at the oil pump that returns the separated oil to the screw compressor. The preferred control system disclosed in the patent comprises a temperature sensor in the discharge duct, just ahead of the oil cooler, and a throttling valve controlled by the sensor and located between the refrigerant pump and the compressor discharge duct. With a positive displacement refrigerant pump driven by a constant speed motor, a pressure relief valve is connected in a return circuit between the outlet and the inlet of the refrigerant pump, to circulate back to its inlet such of its output as is not passed by the throttling valve.
The inclusion of this relief valve added to the cost and complexity of the apparatus, but the relief valve arrangement was nevertheless considered preferable to other obvious expedients for controlling the rate of delivery of liquid refrigerant to the compressor discharge duct. In particular, the use of a variable speed electric motor and means for controlling its speed in response to temperature in the discharge duct would have been more expensive and complicated than provision of the relief valve.
The patent points out that the system poses a problem with respect to adequate seals in the refrigerant pump, inasmuch as the liquid refrigerant bypassed from the high pressure receiver to the discharge duct is maintained under substantially high pressure as it passes through the pump. The refrigerant pump is therefore said to require expensive high pressure seals, but the patent characterizes the relatively high cost of such a pump as "insignificant in relation to the economic benefits achieved with the oil cooling means of the present invention." Thus the need for high pressure seals--although tolerable in view of off-setting gains--was recognized as a real disadvantage and one that had to be accepted because there was no obvious expedient for avoiding it.
Nevertheless, difficulties were encountered in providing completely effective high pressure seals for the refrigerant pump, and leakage through the seals, although not frequent, could occur and had potentially serious consequences when it did occur. Consideration was given to enclosing both the pump and its electric drive motor in a hermetically sealed housing, with no refrigerant seal between the pump and the motor, but this proposal was rejected because it solved one problem at the risk of creating another and more serious one. If the pump drive motor burned out, acids from its overheated insulation would contaminate the entire refigeration system.
Another problem sometimes encountered with the operation of the apparatus of U.S. Pat. No. 4,270,570 was cavitation of its refrigerant pump. The duct communicating the refrigerant pump inlet with the high pressure receiver is of relatively small diameter, because only a small rate of flow of refrigerant has to be produced by that pump. Liquid refrigerant in the receiver is near its vaporizing pressure, and pressure drop along the narrow duct leading to the refrigerant pump sometimes caused bubbles of vaporized refrigerant to form in that duct and cause cavitation at the pump.